Influenza is an acute respiratory infectious disease caused by infection with an influenza virus. In Japan, there is a report of a few millions of influenza-like patients every winter, and influenza is accompanied with high morbidity and mortality. Influenza is a particularly important disease in a high risk population such as baby and elderly, a complication rate with pneumonia is high in elderly, and death with influenza is occupied with elderly in many cases.
As anti-influenza drugs, Symmetrel (trade name: Amantadine) and Flumadine (trade name: Rimantadine) which inhibit the denucleation process of a virus, and Oseltamivir (trade name: Tamiflu) and Zanamivir (trade name: Relenza) which are neuraminidase inhibitors suppressing virus budding and release from a cell are known. However, since problems of appearances of resistant strains and side effects, and worldwide epidemic of a new-type influenza virus having high pathogenicity and mortality are feared, development of an anti-influenza drug having a novel mechanism has been desired.
Since a cap-dependent endonuclease which is an influenza virus-derived enzyme is essential for virus proliferation, and has the virus-specific enzymatic activity which is not possessed by a host, it is believed that the endonuclease is suitable for a target of an anti-influenza drug. The cap-dependent endonuclease has a host mRNA precursor as a substrate, and has the endonuclease activity of producing a fragment of 9 to 13 bases including a cap structure (not including the number of bases of the cap structure). This fragment functions as a primer of a virus RNA polymerase, and is used in synthesizing mRNA encoding a virus protein. That is, it is believed that a substance which inhibits the cap-dependent endonuclease inhibits synthesis of a virus protein by inhibiting synthesis of virus mRNA and, as a result, inhibits virus proliferation.
As the substance which inhibits the cap-dependent endonuclease, flutimide (Patent Document 1 and Non-Patent Documents 1 and 2) and 4-substituted 2,4-dioxobutanoic acid (Non-Patent Documents 3 to 5) are reported, but they have not yet led to clinical use as anti-influenza drugs. In addition, Patent Documents 2 to 11 and Non-Patent Document 6 describe compounds having a similar structure to that of a novel anti-influenza drug exhibiting the cap-dependent endonuclease inhibitory activity, and a method of producing the same. In addition, a pyrone derivative and a pyridone derivative are disclosed in Non-Patent Documents 7 to 9.